NiV3O8 single crystal structure determination and comparison with polymorphic forms of ZnV3O8 and MgV3O8

A low temperature form of NiV3O8 compound was synthesized by solid state chemistry and its structure determined by single crystal X-ray diffraction. NiV3O8 crystallizes in the orthorhombic system with space group Pbcm and cell parameters a=7.2340(6) ?, b= 10.0520(4) ?, c=8.3930(8) ?. The structure is built up with puckered [(Ni2+/V4+)2O7]n layers connected by [V2O7] pyrogroups. Examination of the coordination polyhedra clearly shows that this compound is a pure vanadyl vanadate without a homogeneity range contrary to previous observations. Comparison with other related MV3O8 compounds (M=Zn; Mg) shows that these structures are built up with the same layers but both the way of pilling as well as the way of connection via pyro group are different. These differences lead us to formulate this structure as (delta)NiV3O8 to distinguish it from the low (alpha)and high (beta)temperature form of other MV3O8 compounds. The magnetic behaviour shows two critical temperatures (2 and 27K) where ferromagnetic phases appear.